The present invention is concerned with a novel process for the manufacture of d,l-.alpha.-tocopherol by condensing trimethyl-hydroquinone with isophytol. d,l-.alpha.-Tocopherol is a diastereomer mixture of a derivative of the vitamin E group.
Various processes for the manufacture of d,l-.alpha.-tocopherol by condensing trimethylhydroquinone with isophytol have already been described.
Thus, for example, according to Chem. Abstracts (C.A.) 103, 104799 (1985), C.A. 103, 123731 (1985) and C.A. 110, 39221 (1989) the condensation is carried out in the presence of zinc and zinc chloride (ZnCl.sub.2) and a protonic acid such as a hydrohalic acid, e.g., hydrochloric acid (HCl), trichloroacetic acid, acetic acid and the like, especially ZnCl.sub.2 /HCl, as the catalyst.
The manufacture of d,l-.alpha.-tocopherol by reacting trimethylhydroquinone with phytyl chloride or isophytol in the presence of boron trifluoride (BF.sub.3) or its etherate (BF.sub.3 .multidot.Et.sub.2 O) as the catalyst is described in German Offenlegungsschriften 960720 and 1015446 as well as in U.S. Pat. No. 4,634,781.
The condensation of trimethylhydroquinone with isophytol or phytol, which has been treated with ammonia or amines, in the presence of ZnCl.sub.2 /HCl or a Lewis acid/HCl, such as, e.g., BF.sub.3 or aluminium trichloride (AlCl.sub.3), as the catalyst is described in U.S. Pat. No. 4,634,781.
According to U.S. Pat. No. 3,789,086 the condensation is carried out in the presence of a FeCl.sub.2 /Fe/HCl catalyst, while according to EP-A 12824 the condensation is effected using trifluoroacetic acid.
All of these previously known processes have serious disadvantages: thus, corrosion problems occur in all processes, when boron trifluoride is used there are additionally toxicity problems with the boron trifluoride adducts, and when iron or zinc is used there is a contamination of the waste water with iron or zinc ions which today is no longer acceptable.
The use of ion exchangers, e.g., Amberlyst 15.RTM., as the catalyst is described in U.S. Pat. No. 3 459 773. However, according to this process the d,l-.alpha.-tocopherol could not be obtained in the requisite purity.